1. Introduction
This invention relates to anionic polymeric hydrogels and more particularly to anionic hydrogels, based on sulfonated-amino-aldehyde resins, and to a process for making such hydrogels.
2. Description of the Prior Art
Hydrogels are three dimensional networks of polymers generally covalently or ionically cross-linked and exhibit the ability to swell in water and retain significant amounts of the imbibed water. Hydrogels may be made from a variety of compositions and some are classified as neutral, anionic and cationic hydrogels. Hydrogels can also be prepared by modifying existing polymers and comprise a broad class of polymeric materials which are insoluble, but significantly swollen by water (30-95% by wt). In general, there are six basic classes of hydrogels: (1) poly(hydroxy alkyl methacrylates); (2) poly(acrylamide); poly(methacrylamide) and derivatives; (3) poly(N-vinyl-2-pyrrolidone); (4) anionic and cationic hydrogels; (5) polyelectrolyte complexes; and (6) polyvinyl alcohols.
Many of the known cross-linked hydrogels are neutral and are not ionic in character. However, anionic and cationic hydrogels have also been synthesized and used for biomedical applications. These hydrogels can be obtained by:
(1) Copolymerization of a neutral hydrogel monomer with an anionic or cationic monomer; or
(2) From polyelectrolyte complexes by adding an excess of the polyanion or polycation component.
Poly(N-vinyl 2-pyrrolidone) (P-NVP) has also been used in the preparation of hydrogels. This polymer is water soluble in its uncrossed form. And because of its strong interaction with water, P-NVP can be used for preparing gels which will exhibit high water contents. One of the most important uses for such P-NVP solutions has been as a plasma extender. It is non-toxic and non-thrombogenic. It has also been used as a tablet coating and for the solubilization and stabilization of drugs. However, hydrogels consisting of P-NVP have not often been described in the biomedical literature because high concentrations of cross-linked polymers are needed to produce hydrogels with useful mechanical properties. It is also difficult to prepare homogeneous P-NVP materials. NVP is also considered to be an ideal monomer for use in covalent surface grafting systems. In addition, poly(N-vinyl 2-pyrrolidone) has been copolymerized with poly(hydroxyethyl acrylate or methacrylate) to produce hydrogels for various applications in the biomedical field such as hydrogel contact lenses.
Although hydrogels have been known for many years, it is only within the past ten years, that scientists have recognized the true potential for these polymers. For example, scientists have learned that such materials are compatible with human tissue, and that negatively charged surfaces are less thrombogenic than positively charged ones because blood cells have a negative charge. It was also reported that polyelectrolyte complexes containing excess anionic component were found to have the lowest thrombogenicity in vivo studies. However, applications of hydrogels are usually limited to materials with a water content up to 80%. This is because if the water content is increased further, the decline in mechanical properties is unacceptable for most purposes. This can be overcome to a degree by:
(1) Copolymerization of hydrophilic monomers, with hydrophobic monomers; and
(2) Grafting of hydrophillic monomers on stronger polymeric supports by radiation cross-linking or by dipcoating of the hydrophillic monomer on to the hydrophobic substrate.
Hydrogels have been used in the biomedical field for various applications such as (1) contact lenses, (2) artificial organs, (3) tissue substrates, (4) hemodialysis membranes, (5) burn wound dressing, and (6) drug carriers. They have also been used in the agricultural field. For example, a polyacrylamide copolymer has been developed by Agricultural Polymers Ltd. (UK) as a hydrogel for soil. It is reported that water uptake can go up to a few hundred times the dry weights of the polymer. It was also reported that stress in plants due to a shortage of water can be drastically reduced by the use of such material. Another example is the use of radiation cross-linked poly(ethylene oxide) by Union Carbide as a soil modifier. This product is being distributed as Viterra I and II. However, the use of hydrogel materials for agricultural purposes are reported to be economically unattractive.
The fact that the low-molecular-weight compounds can diffuse through swollen hydrophillic gels can also be used to achieve a slow release physiologically active substance incorporated in the gel. As a result, active species of different functions can be complexed with these hydrogels. Such hydrogel forming polymers work by virtue of their polyhydroxyl functionality (--OH, C--O--C, --C.dbd.O), and are therefore used as polymeric inert carriers which control the release from these carriers. The active substances may be drugs, agricultural chemicals or fragrances. Hydrogels are also used as antifogging coatings.
Another application for polymeric hydrogels is for ultrafiltration and reverse osmosis (RO) membranes. RO membranes prepared from polyelectrolyte complex resins were found to display fluxes about three orders of magnitude higher than those made from cellulose acetate. These menbranes exhibit a striking rejection of calcium and other divalent ions and have been evaluated as saline water pretreatment filters in membrane desalination to prevent buildup of a fouling layer on the surface of cellulose acetate membrane.
Thus, there is a need for new polymeric hydrogels to meet the demand for more economical and more versatile products. Accordingly, a new composition of hydrogels having broader applications has now been developed. Such hydrogels can be economically produced, are nontoxic and are believed to be suitable for use as a source of slow release fertilizer in addition to their function as a water-reservoir.